Double-polyelectrolyte, like-charged amphiphilic diblock copolymers: swollen structures and pH- and salt-dependent lyotropic behavior.

We consider a symmetrical poly(styrene- stat-(acrylic acid))- block-poly(acrylic acid), i.e., PSAA- b-PAA, diblock copolymer, with a molar fraction phi AA = 0.42 of acrylic acid, in the more hydrophobic PSAA statistical first block. We investigate its structural behavior at constant concentration in water using small-angle neutron scattering (SANS) by varying (i) the ionization of its acrylic acid motives via the pH by adding NaOH and (ii) the ionic strength of the solution by increasing the NaCl salt concentration c S. We present the resulting morphological phase diagram {pH, c S}, in which we identified two different lamellar phases presenting a smectic long-range order at small-to-intermediate ionizations and a spherical phase with a liquid-like short-range order at larger ionization. In the low-ionization regime, the first lamellar phase comprises a water-free PSAA lamellar core surrounded by a dense poly(acrylic acid) brush swollen with water. Its mostly hydrophobic core still being glassy, this phase is unable to reorganize and is frozen in. A detailed analysis of the SANS data shows the osmotic nature of the polyelectrolyte brush, in which the Na+ counterions are confined so that local electroneutrality is satisfied. Above the pH at which the PSAA statistical block starts ionizing, the PSAA lamellar core melts. The second lamellar phase identified then comprises a PSAA core thinner than that of the frozen-in previous phase, implying a significant increase of the core/water interface and a decrease of the brush surface density. The transition from the first lamellar phase to the second one can be quantitatively shown to result from the balance between the two contributions: (i) the extra interfacial cost between the thinner core and water and (ii) the associated gain in entropy of mixing for the counterions confined inside the brush. At even higher ionization, the diblocks finally form spherical objects with a very small, pH-dependent aggregation number and reach an apparent onset of self-association. When the highest ionization investigated is reached, the cores of these final spherical core-shell objects are found to contain a significant amount of water. We thereby demonstrate that at constant concentration, pH, and ionic strength both trigger a transition from frozen to molten hydrophobic phases as well as unexpected morphological transitions.

Interactions between sulfobetaine-based polyzwitterions and polyelectrolytes.

We investigate the interactions between sulfobetaine-based polyzwitterions and polyelectrolytes, either positive or negative ones, i.e., poly(DADMAC)s and poly(AA)s. Three different sulfobetaine motifs denoted SPE, SPP, and SHPP have been considered, presenting slight chemical changes either in the function carrying the zwitterionic group or in the zwitterionic motif itself. All three poly(sulfobetaine)s normally present critical temperatures (T(c)) above which they become fully soluble. The association with polyelectrolytes directly affects the critical temperature in a highly nonmonotonic fashion as the mixture composition is varied. Thanks to layer-by-layer deposition in a reflectometric cell, we demonstrate that a selective attraction exists between polyzwitterions and polyelectrolytes, from which an association follows at a nanoscopic scale as demonstrated by small-angle X-ray scattering and atomic force microscopy. The association of polyzwitterions with polyelectrolytes, however, is site-specific since it exists only between positive polyelectrolytes (i.e., polycations) and polyzwitterions based on SPE or SPP motifs. The range in which the association affects the critical temperature, T(c), is found to largely depend on the molecular weights of both zwitterionic and cationic species. As a result, the complexation and the creation of a hybrid object, referred to as a complex, also depend on the same parameters. By varying the latter from a few thousands to several millions, we define rules for the existence of this complex. In particular, a minimum polyzwitterion molecular weight is needed to observe alterations of the critical temperatures and closure of the complexation cone. Finally, within a Flory-like approach, we consider the polyzwitterion/polyelectrolyte complex as an effective statistical copolymer, whose composition comprises a fraction phi(A) of excess zwitterionic motifs as the majority species and a fraction 1 - phi(A) of complex motifs. We thereby reduce a polymer/polymer/solvent ternary system to a copolymer/solvent binary one, an assumption valid within the limit of small additions of cationic species. The approach predicts the reciprocal critical temperature 1/T(c) to be quadratic in phi(A), which agrees very well with all experimental results, even for a large mismatch between the molecular weights of both species, and regardless of the zwitterionic motif, SPE or SPP.

Reconciling low- and high-salt solution behavior of sulfobetaine polyzwitterions.

We investigate the water-solubility upon salt additions, of homogeneous families of sulfobetaine-based polyzwitterions. These polymers bear both positive ammonium, and negative sulfonate charges on each monomer and as a result present an upper critical solution temperature (UCST) in the 0-100 degrees C temperature range. Two chemistries are investigated, with either a carboxylate-carrying function (SPE) or an amido-carrying one (SPP). In agreement with the literature published on pSPEs, we find that an addition of simple salts improves the water-solubility of pSPEs, as well as that of pSPPs, yet only once a threshold concentration of added salt has been reached in the solution. We verify using scaling arguments that the onset of solubility promotion, corresponds exactly to the complete screening of the attraction between positive and negative charges inside a polyzwitterionic coil. On the contrary, for salt concentrations smaller than the threshold concentration, we observe that an addition of salt can be adverse to the solubility of polyzwitterions, depending on the degree of polymerization, the type of salt, and the type of zwitterionic motive. Thanks to zeta-potential measurements and systematic variations of these three parameters, we demonstrate, in agreement with theoretical prediction, that this molecular weight-dependent enhanced solubility at small salt concentrations is due to charge asymmetry resulting from partial hydrolysis, combined with specific interactions between salts and zwitterion constituents, evidencing the complexity of the solution behavior of these macromolecules. We thereby reconcile the different behaviors in the domains of low- and high-salinity.

Chemically tuned amphiphilic diblock copolymers dispersed in water: from colloids to soluble macromolecules.

We investigate by small-angle scattering the structural behavior in water of a family of asymmetric poly(styrene-stat-(acrylic acid))-block-poly(acrylic acid), i.e., P(S-stat-AA)-b-PAA, diblock copolymers. These diblocks are of constant block ratio and increasing molar fraction, phi(AA), ranging from 0 to 1, of acrylic acid in the first P(S-stat-AA) statistical block. We identify three types of structural behavior in water: (i) for phi(AA) /= 0.50, the diblocks dispersions in water are at equilibrium. For high phi(AA), the diblocks are soluble in water, demonstrating that a transition from colloid-like objects to soluble macromolecules is achieved. Close to the transition, (phi(AA) approximately 0.50), the diblocks form objects interpreted as comprising a water-swollen core formed by the P(S-stat-AA) block, surrounded by a swollen brush composed of the majority PAA block, above a apparent critical micelle concentration. However, these diblocks do not behave as macrosurfactants, and their self-association behavior is rather interpreted as a microphase separation which can arise from the incompatibility between two polymer blocks P(S-stat-AA) and PAA placed in a common solvent water.